Biochemistry: Enzymes, Metabolism, and ATP
Grade 12 · Science · Study Guide
Explore how cells harness energy through metabolic pathways, the role of enzymes as biological catalysts, and how ATP powers life.
All living organisms require energy to carry out cellular processes. Metabolism encompasses all chemical reactions in a cell, divided into catabolism (breaking down molecules to release energy) and anabolism (building molecules using energy). The primary energy currency of the cell is adenosine triphosphate (ATP), produced by cellular respiration and used to power nearly every cellular process from muscle contraction to protein synthesis.
Enzymes are biological catalysts — typically proteins — that dramatically accelerate biochemical reactions by lowering the activation energy. Each enzyme has a specific three-dimensional active site that binds its substrate (the reactant molecule). The enzyme-substrate interaction forms an enzyme-substrate complex (lock-and-key model) or the enzyme changes shape to grip the substrate (induced fit model). Enzyme activity is affected by temperature (optimal ~37°C for human enzymes), pH, substrate concentration, and inhibitors. Competitive inhibitors block the active site; non-competitive inhibitors bind elsewhere and change the active site shape.
Cellular respiration is the key catabolic pathway: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. It occurs in three main stages: glycolysis (cytoplasm, produces 2 ATP), the Krebs cycle (mitochondrial matrix, produces NADH and FADH₂), and the electron transport chain (inner mitochondrial membrane, produces ~32–34 ATP). The complete oxidation of one glucose molecule yields approximately 36–38 ATP. Fermentation occurs when oxygen is absent, regenerating NAD⁺ from NADH to allow glycolysis to continue, but with much lower ATP yield.
| Stage | Location | Inputs | Outputs | ATP yield |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose | 2 Pyruvate, 2 NADH | 2 net ATP |
| Krebs Cycle | Mitochondrial matrix | 2 Acetyl-CoA | CO₂, NADH, FADH₂ | 2 ATP |
| Electron Transport Chain | Inner mitochondrial membrane | NADH, FADH₂, O₂ | H₂O | 32–34 ATP |
Key terms
- ATP (adenosine triphosphate)
- The primary energy carrier in cells; energy is released when the terminal phosphate bond is hydrolyzed.
- Enzyme
- A biological catalyst (usually protein) that speeds up reactions by lowering activation energy without being consumed.
- Active site
- The specific region of an enzyme where the substrate binds and the catalytic reaction occurs.
- Glycolysis
- The first stage of cellular respiration; breaks glucose into two pyruvate molecules; produces 2 net ATP; occurs in cytoplasm.
- Krebs cycle
- Second stage of aerobic respiration in the mitochondrial matrix; produces NADH, FADH₂, CO₂, and 2 ATP per glucose.
- Electron transport chain (ETC)
- Final stage of aerobic respiration; uses NADH and FADH₂ to produce ~32–34 ATP via oxidative phosphorylation.
- Inhibitor
- A molecule that reduces enzyme activity; competitive inhibitors mimic the substrate; non-competitive alter enzyme shape.
Flashcards
- ATP
- Adenosine triphosphate; cell's energy currency; releases energy when terminal phosphate is cleaved
- Enzyme active site
- 3D pocket on enzyme where substrate binds; specific shape creates substrate specificity
- Glycolysis
- Glucose → 2 pyruvate + 2 ATP + 2 NADH; anaerobic; in cytoplasm
- Krebs cycle
- Pyruvate → CO₂ + NADH + FADH₂ + 2 ATP; aerobic; in mitochondrial matrix
- ETC/Oxidative phosphorylation
- NADH + FADH₂ + O₂ → H₂O + ~32-34 ATP; on inner mitochondrial membrane
- Fermentation
- Anaerobic regeneration of NAD⁺; lactic acid or ethanol produced; only 2 ATP total
Practice questions
Where does glycolysis occur in the cell?
- Mitochondrial matrix
- Inner mitochondrial membrane
- Cytoplasm
- Nucleus
Show answer
Answer: Cytoplasm
Glycolysis occurs in the cytoplasm of all cells. It does not require mitochondria, which is why even anaerobic cells can perform this step.
Enzymes are consumed during the reactions they catalyze.
Show answer
Answer: False
Enzymes are biological catalysts — they are not consumed or permanently altered during a reaction. They can catalyze thousands or millions of reactions before breaking down.
The energy currency of the cell is ___.
Show answer
Answer: ATP
ATP (adenosine triphosphate) is the universal energy currency. The hydrolysis of ATP to ADP + Pᵢ releases approximately 30.5 kJ/mol of free energy.
Which stage of cellular respiration produces the most ATP?
- Glycolysis
- Krebs cycle
- Fermentation
- Electron transport chain
Show answer
Answer: Electron transport chain
The ETC (oxidative phosphorylation) uses NADH and FADH₂ produced in earlier steps to drive ATP synthase, producing approximately 32–34 of the ~36–38 total ATP from glucose.
A competitive inhibitor binds to the active site of an enzyme, blocking the substrate.
Show answer
Answer: True
Competitive inhibitors have a similar shape to the substrate and block the active site, preventing the actual substrate from binding. Increasing substrate concentration can overcome competitive inhibition.
Fun facts
- Your body recycles its own weight in ATP every single day — a typical adult produces and uses about 40 kg of ATP, but the total amount in your body at any moment is only about 250 grams.
- Some enzymes are incredibly efficient — catalase can break down over 40 million molecules of hydrogen peroxide per second.
- Cyanide is lethal because it blocks the final enzyme in the electron transport chain, stopping ATP production almost instantly.
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